1. Field of the Invention
The invention relates to a discharge lamp which is advantageously used, for example, as a light source of a liquid crystal display device or the like.
2. Description of Related Art
As the light source part of a liquid crystal display device of the projection type or the like, a discharge lamp is used which has a discharge vessel consisting of silica glass and which has a spherical or oval arc tube portion and hermetically sealed portions which are located bordering the two ends of this arc tube portion. In this discharge lamp, furthermore, there is a pair of opposed electrodes in the arc tube portion, and the electrode rods of these electrodes are connected to molybdenum metal foils (hereinafter also called xe2x80x9cmolybdenum foilsxe2x80x9d) which are installed in the hermetically sealed portions and which form electrical feed bodies. In this discharge lamp, hermetically sealed areas are formed by the directly adjoining tight arrangement produced by melting of the silica glass which forms the hermetically sealed portions onto the surfaces of these molybdenum foils.
However, in a discharge lamp with these hermetically sealed areas, on the boundary surfaces between the molybdenum foils as the conductive bodies and the silica glass as the nonconductive body there is a large potential gradient. This results in the phenomenon that cations such as alkali ions or the like collect as impurities in the silica glass in the vicinity of the boundary surface to the molybdenum foils. When the discharge lamp is shifted into the sealed state and when it reaches a high temperature state, in the silica glass comprising the hermetically sealed portions, crystallization nuclei are formed, for example, by the cations. As a result, a phase conversion occurs and a crystal body, such as quartz, cristobalite or the like is formed. One such conversion into a crystal body makes nonuniform the boundary surface structure between the molybdenum and the silica glass which has been produced by the sealing process, and diminishes the mechanical strength. Cracks form in the silica glass comprising the hermetically sealed portions, therefore, proceeding from the locations of the boundary surfaces to the molybdenum foils, by which the sealing action in the hermetically sealed areas is lost. Finally, there was the disadvantage that the expected service life of the discharge lamp cannot be obtained.
The silica glass and the molybdenum foils are joined to one another by a physical force, the penetration of silica glass into the concave parts and convex parts of the surfaces of the molybdenum foils and by a chemical force which is formed by the chemical bonding of the two. However, this chemical bond is destroyed by the attack of an alkali metal or alkali halogenide. The adhesive strength between the silica glass and the molybdenum foils therefore gradually decreases; this leads to detachment of the molybdenum foils from the silica glass. For this reason, the sealing action in the hermetically sealed areas is gradually lost. Finally, there was the disadvantage that the expected service life of the discharge lamp cannot be maintained.
The invention was devised to eliminate the above described defects in the prior art. Thus, an object of the invention is to devise a discharge lamp which has hermetically sealed areas using metal foils in which the endurance in the above described hermetically sealed areas is high and in which a long service life is obtained as a result.
In a discharge lamp which has a silica glass discharge vessel which has an arc tube portion in which there is a pair of opposed electrodes and which has hermetically sealed portions which are located on the ends of this arc tube portion, in which in the hermetically sealed portions of this discharge vessel molybdenum metal foils which form electrical feed bodies are installed, and in which, thus, hermetically sealed areas are formed, the object is achieved in accordance with the invention in that, at least on one side of the above described respective metal foil, a coating layer is formed from at least one metal oxide which is selected from titanium oxide, lanthanum oxide and tantalum oxide.
Furthermore, in a discharge lamp which has a silica glass discharge vessel which has an arc tube portion in which there is a pair of opposed electrodes, and which has hermetically sealed portions which are located on the ends of this arc tube portion, in which, in the hermetically sealed portions of this discharge vessel, molybdenum metal foils which form electrical feed bodies are installed, and in which hermetically sealed areas are formed, the object is achieved according to the invention in that, at least on one side of the above described respective metal foil, a coating layer is formed from at least one metal oxide which is selected from zirconium dioxide which contains 0% by mole to 20% by mole yttrium oxide, and hafnium oxide which contains 0% by mole to 40% by mole yttrium oxide.
Furthermore, in accordance with the invention, the object is advantageously achieved in the above described arrangement in that the metal oxide which forms the coating layer is crystalline.
Furthermore, according to the invention, the object is advantageously achieved in the above described arrangement in that the coating layer is formed over a base layer which is made of aluminum oxide or yttrium oxide, at least on one side of the above described respective metal foil.
In the discharge lamp of the invention, at least one side of the respective molybdenum metal foil is surrounded by a coating layer in the hermetically sealed area which forms the electrical feed body. For this reason the alkali metal cations and the like which are present as impurities in the silica glass comprising the hermetically sealed portions move into the vicinity of the metal foils and collect there. By coating the metal foils with coating layers which consist of a certain metal oxide, deterioration of the characteristic by the effect of the cations is prevented. Moreover a phase conversion in the silica glass which is caused by the accumulation of cations is prevented. As a result, very high endurance of the hermetically sealed areas in these hermetically sealed portions is obtained. As a result a long service life in the discharge lamp can be obtained.
The invention is further described below using the accompanying drawings.